In article , Essjay001
writes

Yes burn them it is not clever and probably illegal to plant non native
species that have not been properly certified. You have no idea if they are
disease free, you could be planting a time bomb. Remember a time whem this
country was covered in Elm Trees!

The following DEFRA page is not crystal clear, but states that the
import of most (flower) seeds is unrestricted.

http://www.defra.gov.uk/planth/travel.htm
--
Stewart Robert Hinsley